We propose to continue our studies of the molecular conformations of prostaglandins via the technique of single crystal X-ray diffraction. The goal of these studies is the elucidation of the relationship of prostaglandin configuration to prostaglandin conformation and our working hypothesis is that the configuration of a prostaglandin at six critical loci directs the conformation of the prostaglandin. Accordingly, we have outlined the proposed study of both naturally occurring prostaglandins and their analogs, selected specifically to probe each of these loci. An empirical relationship of this kind can provide a rational basis for the search for structure-activity or structure-binding relationships by yielding relevant structural data without the need for detailed crystallographic, spectral, or theoretical analyses; and provide a practical rationale for the design of specific analogs with specific conformation features. In connection with our continued studies of the prostaglandins, we propose to expand our conformational studies to the closely related prostaglandin endoperoxides, thromboxanes and prostacyclins (or analogs thereof). All of these compounds are biologically derived from precursor acids common the natural prostaglandins, and their biological activities are closely related to the prostaglandins. Our aim here is to determine the similarities and differences in molecular conformations for these compounds vis-a-vis the classical prostaglandins. The abundance of putative physiological roles for the prostaglandins and related compounds, particularly their probable involvement in blood pressure regulation, platelet aggregation, and cardio-pulmonary processes, suggests that these compounds will be exploited pharmacologically. We believe that the specificity and activity of prostaglandins are due to their conformational characteristics. Therefore, a relationship between configuration and conformation will be a useful tool in the search for, and understanding of, new drugs related to these potent molecules.